Beyond the Dragon Arc a treasure trove of unseen stars
The galaxy, referred to as the Dragon Arc, is positioned behind a massive galaxy cluster called Abell 370. The immense gravitational field of Abell 370 warps and magnifies the light from the Dragon Arc, stretching its spiral shape into an elongated arc and allowing astronomers to resolve individual stars within the distant galaxy.
"To us, galaxies that are very far away usually look like a diffuse, fuzzy blob," explained lead author Yoshinobu Fudamoto, an assistant professor at Chiba University in Japan and visiting scholar at the University of Arizona's Steward Observatory. "But actually, those blobs consist of many, many individual stars. We just can't resolve them with our telescopes."
Gravitational lensing, predicted by Albert Einstein, occurs when a massive object's gravity bends the light from objects behind it, effectively magnifying them. This natural telescope has enabled the detection of individual stars in distant galaxies, a feat previously unattainable.
"These findings have typically been limited to just one or two stars per galaxy," Fudamoto noted. "To study stellar populations in a statistically meaningful way, we need many more observations of individual stars."
The discovery was serendipitous. Fengwu Sun, a former University of Arizona graduate student and now a postdoctoral scholar at the Center for Astrophysics | Harvard and Smithsonian, was examining JWST images of the Dragon Arc when he identified 44 individual stars whose brightness varied over time due to changes in the gravitational lensing effect.
"This groundbreaking discovery demonstrates, for the first time, that studying large numbers of individual stars in a distant galaxy is possible," Sun said.
The study, published in the journal Nature Astronomy, not only sets a record for the number of individual stars detected in the distant universe but also opens new avenues for investigating dark matter, one of the universe's greatest mysteries.
"Inside the galaxy cluster, there are many stars floating around that are not bound by any galaxy," said co-author Eiichi Egami, a research professor at Steward Observatory. "When one of them happens to pass in front of the background star in the distant galaxy along the line of sight with Earth, it acts as a microlens, in addition to the macrolensing effect of the galaxy cluster as a whole."
This combination of macrolensing by the galaxy cluster and microlensing by individual stars has provided an unprecedented glimpse into the stellar populations of a galaxy billions of light-years away, offering insights into the formation and evolution of galaxies across cosmic time.
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